1. Technical Field
This invention relates generally to a visual based process control apparatus and method for electrostatographic copiers and/or printers.
2. Background Art
In electrophotographic copying and printing a color original document, several factors inhibit perfect and constant color reproduction in terms of color balance, color fidelity, and tone reproduction. These factors include variation in the color and intensity of the light source used to illuminate the document, variation in the spectral reflectance of the different colorants of the document, non-ideal color separation filters, variation in the photoresponse of the photoconductor, variation in the toning contrast in the different color development stations, variations in the transfer or fusing efficiencies, and unwanted absorptions in the colored toners.
Prior art systems attempt to diminish the adverse effects of these factors using manual or automatic setup systems, or a combination of both. In typical manual setup systems, a skilled operator examines the output reproduction (copy or print) and the corresponding input and output density (D.sub.in -D.sub.out) curves for red, green, blue, and black. Based on experience with the equipment, the operator determines adjustments to process control parameters, such as initial voltage V.sub.O, exposure E.sub.O, and development bias V.sub.b. Several iterations of adjustment may be required to achieve acceptable color reproductions, in terms of color balance, color fidelity, and tone reproduction.
The setup procedure is complicated by the fact that a process control parameter adjustment which is favorable to one region of the reproduction may be detrimental for another. For example, a particular color may be too dark at high density levels and too light at low density levels. Accordingly, an adjustment to lighten the color to correct for high density errors would compound the low density error.
During setup, a skilled operator will generally image a neutral density step tablet and adjust the process for hue neutrality of the reproduction. After achieving reasonable neutrality, the operator will adjust for good tone reproduction (i.e., good light-to-dark progression, contrast, and absence of abrupt density changes between density steps). Finally, the operator will check and adjust for neutrality again, all this in an iterative procedure until satisfied with the overall resultant reproduction quality. The setup inevitably involves compromises over all the color areas in the print, and even a highly skilled operator may be unable to achieve acceptable color reproduction within a reasonable time period.
Color copiers and printers are known which include automatic setup means for adjusting one or more of the process control parameters affecting the output color and density. Such automatic adjustment is typically based on density measurements of toned test patches for each color separation independently of the other color separations. However, the human observer is critical not only to the appearance of individual colored areas taken individually, but also judges overall color and tone scale quality by the relationship of one color to another. For example, a slight hue error might be acceptable if the error is uniform over the entire image, but it would be unacceptable if the error is either (1) in one direction in some colors and in another direction in other colors or (2) in one direction for some densities and in another direction for other densities of the same color.
Many automatic systems that adjust process control parameters are responsive to a set of density steps developed on the photoconductive receiver and read by a transmission or reflection densitometer. Alternatively, reflection density steps transferred onto copy paper may be used as the control variable. Only a very small number of density step levels can be sampled relative to the number of gray levels available in, say, a seven bit, 128 step system. Therefore, at least some of the samples will not be at the most desirable density. As such, the error will not be reducible to zero, and the visual effect will not be best.
Different points on the density curve actually have different visual importance to the human eye, which does not respond linearally to density changes. For example, a given variation in density from a desired value will have a much larger effect at a lower density than that at higher density. It has been suggested to calculate process control parameter adjustments utilizing known average human relative visual sensitivities to density and color shifts at various density levels, and to more heavily weight those colors or densities which are particularly important to the scene; See commonly assigned, copending U.S. patent application Ser. No. 189,091, filed on May 2, 1988, in the name of A. Rushing now U.S. Pat. No. 4,853,738 which issued on Aug. 1, 1989.
While the appearance of related colors is strongly dependent on their relative densities, a uniform linear scale of relative densities between two low density samples is much greater than that between two high density samples.